In the past, you were able to set your ads to rotate on based on "optimize for clicks," "optimize for conversions," or "rotate evenly." Based on those 30 days or rotation, Google will pick one and keep it for that ad.

The issue is, advertisers like the control.

Brad Geddes, an expert PPCer, posted on Google+ a possible workaround that he heard through the grapevine.

He said that when you pause your ads and then unpause them, the ad rotation 30-day cycle goes back to day one. So he suggested you use the AdWords API and set a script to pause and unpause ads every 29 days or so.

He said he has a highly credible source at Google who said:

I've confirmed with multiple sources including [withdrawn] that any addition or edit in an adgroup, including un-pausing or un-deleting an ad, will reset the clock. There's no need to let them run for some certain amount of time; it is reset immediately.